U.S. patent application number 10/865101 was filed with the patent office on 2004-11-11 for process boat and shell for wafer processing.
Invention is credited to Galambos, Ludwig L., Lazar, Janos J., Miles, Ronald O..
Application Number | 20040221810 10/865101 |
Document ID | / |
Family ID | 33544447 |
Filed Date | 2004-11-11 |
United States Patent
Application |
20040221810 |
Kind Code |
A1 |
Miles, Ronald O. ; et
al. |
November 11, 2004 |
Process boat and shell for wafer processing
Abstract
In one embodiment, an apparatus for wafer processing comprises a
boat and a shell. The shell may be configured to receive and
enclose the boat, which in turn may be configured to receive a
plurality of wafers. The shell may include a plurality of slots to
allow vapor to escape out of the shell and away from the wafers
during a temperature ramp down. The apparatus may be employed in a
variety of wafer processing applications including in processes for
increasing the bulk conductivity of ferroelectric materials, for
example.
Inventors: |
Miles, Ronald O.; (Menlo
Park, CA) ; Galambos, Ludwig L.; (Menlo Park, CA)
; Lazar, Janos J.; (Redwood City, CA) |
Correspondence
Address: |
OKAMOTO & BENEDICTO, LLP
P.O. BOX 641330
SAN JOSE
CA
95164
US
|
Family ID: |
33544447 |
Appl. No.: |
10/865101 |
Filed: |
June 9, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10865101 |
Jun 9, 2004 |
|
|
|
10187330 |
Jun 28, 2002 |
|
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|
60480566 |
Jun 20, 2003 |
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Current U.S.
Class: |
118/715 |
Current CPC
Class: |
C01G 33/00 20130101;
C01G 35/00 20130101; H01L 21/67313 20130101; C01P 2006/42 20130101;
H01L 21/67326 20130101; H01L 21/67109 20130101 |
Class at
Publication: |
118/715 |
International
Class: |
A61M 036/14; A61K
051/00; C23C 016/00 |
Claims
What is claimed is:
1. A wafer processing apparatus comprising: a boat configured to
receive a plurality of wafers; and a shell configured to receive
and enclose the boat and a metal source, the shell being configured
to contain vapor of the metal source in a vicinity of the plurality
of wafers during a main portion of a process, the shell including a
plurality of slots configured to allow vapor of the metal source to
escape out of the shell and away from the plurality of wafers
during a temperature ramp down.
2. The apparatus of claim 1 wherein the shell comprises a top
portion and a bottom portion that may be joined together to enclose
the boat.
3. The apparatus of claim 1 wherein the vapor comprises zinc
vapor.
4. The apparatus of claim 1 wherein the boat and the shell form a
cage configured to be heated in a process tube.
5. The apparatus of claim 1 wherein the boat comprises: a plurality
rods having one or more notches and configured to receive an edge
of a wafer in the plurality of wafers; a plurality of U-pieces
attached to the rods to form a structure for holding the plurality
of wafers; and a plurality of bar pieces configured to facilitate
handling of the boat.
6. The apparatus of claim 5 wherein the shell comprises a top
portion and a bottom portion that each has a clearance for
accepting a bar piece in the plurality of bar pieces.
7. The apparatus of claim 6 wherein the top portion and the bottom
portion have a mating socket and prong connection.
8. The apparatus of claim 1 wherein the plurality of wafers
comprise lithium tantalate wafers.
9. The apparatus of claim 1 wherein the boat and the shell are made
of quartz.
10. A wafer processing apparatus comprising: boat means for
receiving a plurality of wafers; and shell means for receiving and
enclosing the boat and a metal source, the shell means including
slots to allow a vapor of a metal source to escape out of the shell
during a temperature ramp down.
11. The wafer processing apparatus of claim 10 wherein the shell
means comprises a top portion having the slots and a bottom portion
supporting the boat.
12. The wafer processing apparatus of claim 10 wherein boat means
and the shell means form a cage configured to be heated in a
process tube.
13. The wafer processing apparatus of claim 10 wherein the
plurality of wafers comprise lithium tantalate.
14. The wafer processing apparatus of claim 10 wherein the metal
source comprises zinc.
15. The wafer processing apparatus of claim 10 wherein the boat
means and the shell means are made of quartz.
16. An apparatus for wafer processing comprising: a shell having a
top portion and a bottom portion, the top portion including a
plurality of slots to allow vapor to escape out of the boat
substantially through the slots during a temperature ramp down; and
a boat configured to support a plurality of wafers, the boat being
configured to be enclosed by the shell during processing of the
wafers.
17. The apparatus of claim 16 wherein the boat further comprises: a
plurality rods having one or more notches and configured to receive
an edge of a wafer in the plurality of wafers; a plurality of
U-pieces attached to the rods to form a structure for holding the
plurality of wafers; and a plurality of bar pieces configured to
facilitate handling of the boat.
18. The apparatus of claim 16 wherein the boat and the shell form a
cage configured to be heated in a process tube.
19. The apparatus of claim 16 wherein the plurality of wafers
comprise lithium tantalate.
20. The apparatus of claim 16 wherein the shell and the boat
comprise quartz.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S.
application Ser. No.10/187,330, filed on Jun. 28, 2002, entitled
"Method And Apparatus For Increasing Bulk Conductivity Of A
Ferroelectric Material," which is incorporated herein by reference
in its entirety.
[0002] This application claims the benefit of U.S. Provisional
Application No. 60/480,566, filed on Jun. 20, 2003, entitled
"Process Boat And Shell For Wafer Processing," which is
incorporated herein by reference in its entirety.
[0003] This application is related to U.S. application Ser. No.
______, filed on the same day as this application by Ludwig L.
Galambos, Joe McRae, and Ronald O. Miles, entitled "Method And
Apparatus For Increasing Bulk Conductivity Of A Ferroelectric
Material," Attorney Docket No. 10021.001321 (P0314), which is
incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
[0004] 1. Field of the Invention
[0005] The present invention relates generally to material
processing, and more particularly, but not exclusively, to methods
and apparatus for processing a ferroelectric material.
[0006] 2. Description of the Background Art
[0007] Lithium tantalate (LiTaO.sub.3) and lithium niobate
(LiNbO.sub.3) are widely used materials for fabricating nonlinear
optical devices because of their relatively large electro-optic and
nonlinear optical coefficients. These nonlinear optical devices
include wavelength converters, amplifiers, tunable sources,
dispersion compensators, and optical gated mixers, for example.
Lithium tantalate and lithium niobate are also known as
ferroelectric materials because their crystals exhibit spontaneous
electric polarization.
[0008] Because lithium tantalate and lithium niobate materials have
relatively low bulk conductivity, electric charge tends to build up
in these materials. Charge may build up when the materials are
heated or mechanically stressed. Because the charge may short and
thereby cause a device to fail or become unreliable, device
manufacturers have to take special (and typically costly)
precautions to minimize charge build up or to dissipate the
charge.
[0009] The bulk conductivity of a lithium niobate material may be
increased by heating the lithium niobate material in an environment
including a reducing gas. The reducing gas causes oxygen ions to
escape from the surface of the lithium niobate material. The
lithium niobate material is thus left with excess electrons,
resulting in an increase in its bulk conductivity. The increased
bulk conductivity prevents charge build up.
[0010] Although the just described technique may increase the bulk
conductivity of a lithium niobate material under certain
conditions, the technique is not particularly effective with
lithium tantalate. A technique for increasing the bulk conductivity
of a lithium tantalate material is desirable because lithium
tantalate is more suitable than lithium niobate for some
high-frequency surface acoustic wave (SAW) filter applications, for
example.
SUMMARY
[0011] In one embodiment, an apparatus for wafer processing
comprises a boat and a shell. The shell may be configured to
receive and enclose the boat, which in turn may be configured to
receive a plurality of wafers. The shell may include a plurality of
slots to allow vapor to escape out of the shell and away from the
wafers during a temperature ramp down. The apparatus may be
employed in a variety of wafer processing applications including in
processes for increasing the bulk conductivity of ferroelectric
materials, for example.
[0012] These and other features of the present invention will be
readily apparent to persons of ordinary skill in the art upon
reading the entirety of this disclosure, which includes the
accompanying drawings and claims.
DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 shows a schematic diagram of a container in
accordance with an embodiment of the present invention.
[0014] FIG. 2 shows a schematic diagram of a housing in accordance
with an embodiment of the present invention.
[0015] FIG. 3 shows a system for increasing the bulk conductivity
of a ferroelectric material in accordance with an embodiment of the
present invention.
[0016] FIG. 4 shows a flow diagram of a method of increasing the
bulk conductivity of a ferroelectric material in accordance with an
embodiment of the present invention.
[0017] FIG. 5 shows a schematic diagram of a wafer cage in
accordance with an embodiment of the present invention.
[0018] FIG. 6 shows a manufacturing specification for a process
boat in accordance with an embodiment of the present invention.
[0019] FIG. 7 shows a manufacturing specification for a shell in
accordance with an embodiment of the present invention.
[0020] FIG. 8 shows a schematic diagram of a container in
accordance with an embodiment of the present invention.
[0021] FIG. 9 shows a system for increasing the bulk conductivity
of a ferroelectric material in accordance with an embodiment of the
present invention.
[0022] FIG. 10 shows a flow diagram of a method of processing a
ferroelectric material in accordance with an embodiment of the
present invention.
[0023] The use of the same reference label in different drawings
indicates the same or like components. Drawings are not necessarily
to scale unless otherwise noted.
DETAILED DESCRIPTION
[0024] In the present disclosure, numerous specific details are
provided such as examples of apparatus, process parameters, process
steps, and materials to provide a thorough understanding of
embodiments of the invention. Persons of ordinary skill in the art
will recognize, however, that the invention can be practiced
without one or more of the specific details. In other instances,
well-known details are not shown or described to avoid obscuring
aspects of the invention.
[0025] Moreover, it should be understood that although embodiments
of the present invention will be described in the context of
lithium tantalate, the present invention is not so limited. Those
of ordinary skill in the art can adapt the teachings of the present
invention to increase the bulk conductivity of other ferroelectric
materials such as lithium niobate, for example.
[0026] In accordance with an embodiment of the present invention,
the bulk conductivity of a ferroelectric material may be increased
by placing the material in an environment including metal vapor and
heating the material to a temperature up to the Curie temperature
of the material. Generally speaking, the Curie temperature of a
ferroelectric material is the temperature above which the material
loses its ferroelectric properties. By heating a single domain
ferroelectric material to a temperature below its Curie temperature
in the presence of a metal vapor with relatively high diffusivity,
the ferroelectric domain state of the ferroelectric material is not
appreciably degraded.
[0027] Preferably, the metal to be converted to vapor has
relatively high diffusivity and has the potential to reduce the
oxidation state of the ferroelectric material. The inventors
believe that these properties will allow ions of the metal to
diffuse a few microns into the surface of the ferroelectric
material to fill lattice site vacancies, reducing the state of
oxidation and thereby liberating electrons from the ferroelectric
material and beginning a process of filling negative ion site
vacancies throughout the bulk of the material. The electrons that
fill these negative ion site vacancies are believed to be bound to
point defect sites. These bound electrons, in general, will have a
spectrum of energy levels that leave the ferroelectric material
with a distinctive broad coloration. With the filling of lattice
site vacancies and supplying neutralizing electrons to point defect
sites , excess charge can be rapidly neutralized or conducted away
perhaps as a polaron. When excess charge (electron) is introduced
into the. lattice, it is energetically favorable for the electron
to move as an entity within the polarization of the lattice. Such
an entity, referred to as a "polaron", results in increased
electron mobility. Since the electron charge is screened by the
lattice, polarons may move unobstructed by electrostatic forces
along the lattice.
[0028] In one embodiment, the metal to be converted to vapor
comprises zinc and the ferroelectric material comprises lithium
tantalate in wafer form. Zinc vapor may be created by heating zinc
to a temperature slightly below the Curie temperature of the
lithium tantalate wafer. To obtain a vapor pressure that is high
enough for efficient diffusion at a temperature below the Curie
temperature, the metal and lithium tantalate wafer may be heated in
a sealed container that has a predetermined volume. The inventors
believe that heating a lithium tantalate wafer in zinc vapor causes
zinc to diffuse into the surface of the lithium tantalate wafer and
fill lithium site vacancies. It is believed that this results in
the release of extra electrons according to equation 1:
Zn+VLi.sup.-.dbd.Zn.sup.+2Li+2e.sup.- EQ. 1
[0029] It is believed that the extra electrons are trapped in
negative ion site vacancies in the bulk of the lithium tantalate
wafer. Increased electron mobility.in the bulk of the lithium
tantalate wafer results when excess charge build up due to
pyroelectric or piezoelectric effects are conducted away as
polarons . That is, the inventors believe that the increased
conductivity of the lithium tantalate wafer appears to be polaron
in nature.
[0030] Referring now to FIG. 1, there is shown a schematic diagram
of a container 210 in accordance with an embodiment of the present
invention. Container 210 may be used to hold one or more wafers 201
to be processed and a metal 202 to be converted to vapor. Container
210 includes a body 211 and an end-cap 212. End-cap 212 may be
welded onto body 211 using an oxygen-hydrogen torch, for
example.
[0031] Body 211 includes a tube section 213 and a tube section 214.
Container 210 may be sealed by capping tube sections 213 and 214,
and welding end-cap 212 onto body 211. Tube section 214 may be
capped by inserting a plug 215 into tube section 214 and welding
the wall of plug 215 to that of tube section 214. Tube section 213
may be a sealed capillary tube. A vacuum pump may be coupled to
tube section 214 to evacuate container 210. A sealed tube section
213 may be cracked open at the end of a process run to increase the
pressure in container 210 (e.g., to bring the pressure in container
210 to atmospheric pressure).
[0032] Still referring to FIG. 1, one or more wafers 201 may be
placed in a wafer cage 203, which may then be inserted into
container 210. A metal 202 may be placed inside wafer cage 203
along with wafers 201. Wafer cage 203 may be a commercially
available wafer cage such as of those available from LP Glass, Inc.
of Santa Clara, Calif. Wafer cage 203 may be made of quartz, for
example.
[0033] Table 1 shows the dimensions of a container 210 in one
embodiment. It is to be noted that container 210 may be scaled to
accommodate a different number of wafers.
1TABLE 1 (REFER TO FIG. 1) Dimension Value (mm) D1 Inside Diameter
120.00 D2 Outside Diameter 125.00 D3 217.00 D4 279.24 D5 76.20 D6
80.00 D7 40.00 D8 60.00 D9 25.40 D10 Inside Diameter 4.00 Outside
Diameter 6.00 D11 Inside Diameter 7.00 Outside Diameter 9.00
[0034] FIG. 2 shows a schematic diagram of a housing 220 in
accordance with an embodiment of the present invention. Housing 220
may be a cylindrical container made of alumina. Container 210 may
be inserted in housing 220, as shown in FIG. 2, and then heated in
a process tube, as shown in FIG. 3. Housing 220 surrounds container
210 to allow for uniform heating of container 210. Additionally,
housing 220 serves as a physical barrier to protect container 210
from breaking.
[0035] As shown in FIG. 2, housing 220 may have a closed-end 224
and an open-end 221. Container 210 is preferably placed inside
housing 220 such that end-cap 212 is towards open-end 221. Open-end
221 allows for convenient removal of container 210 from housing
220. Open-end 221 also facilitates creation of a thermal gradient
in container 210 during a temperature ramp down. The thermal
gradient results in a cold spot in end-cap 212 that attracts
precipitating metal vapor away from the wafers inside container
210. This minimizes the amount of precipitates that have to be
removed from the surface of the wafers. This aspect of the present
invention will be further described below.
[0036] FIG. 3 shows a system 300 for increasing the bulk
conductivity of a ferroelectric material in accordance with an
embodiment of the present invention. System 300 includes a process
tube 310 containing housing 220. As mentioned, housing 220 houses
container 210, which in turn holds metal 202 and wafers 201.
Process tube 310 may be a commercially available furnace generally
used in the semiconductor industry. Process tube 310 includes
heaters 303 (i.e., 303A, 303B, 303C) for heating housing 220 and
all components in it. Process tube 310 may be 72 inches long, and
divided into three 24-inch heating zones with the middle heating
zone being the "hot zone". Process tube 310 may include a first
heating zone heated by a heater 303A, a second heating zone heated
by a heater 303B, and a third heating zone heated by a heater 303C.
Process tube 310 also includes a cantilever 302 for moving housing
220, and a door 301 through which housing 220 enters and leaves the
process tube. Housing 220 may be placed in the middle of process
tube 310 with open-end 221 facing door 301.
[0037] FIG. 4 shows a flow diagram of a method 400 for processing a
ferroelectric material in accordance with an embodiment of the
present invention. Method 400 will be described using container
210, housing 220, and system 300 as an example. It should be
understood, however, that flow diagram 400, container 210, housing
220, and system 300 are provided herein for illustration purposes
and are not limiting.
[0038] In step 402 of FIG. 4, metal 202 and one or more wafers 201
are placed in wafer cage 203. Wafer cage 203 is then placed inside
container 210. In one embodiment, wafers 201 are 42 degree
rotated-Y lithium tantalate wafers that are 100 mm in diameter,
while metal 202 comprises zinc that is 99.999% pure. In one
embodiment, five wafers 201 are placed in wafer cage 203 along with
about 8 grams of zinc. The zinc may be in pellet form. Zinc pellets
that are 99.999% pure are commercially available from Johnson
Matthey, Inc. of Wayne, Pennsylvania. Note that the amount of zinc
per wafer may be varied to suit specific applications.
[0039] In step 404, container 210 is pumped down to about 10.sup.-7
Torr and then heated to about 200.degree. C. for about five hours.
Step 404 may be performed by welding end-cap 212 onto body 211,
capping tube section 213, coupling a vacuum pump to tube section
214, and heating container 210 with a heating tape wrapped around
container 210. Step 404 helps remove oxygen sources, water, and
other contaminants out of container 210 before metal 202 is
melted.
[0040] In step 406, container 210 is back-filled so that the
pressure in container 210 at slightly below Curie temperature is
approximately 760 Torr. In one embodiment, container 210 is
back-filled to about 190 Torr. This increases the pressure inside
container 210, thus making it safer to heat container 210 to
elevated temperatures for long periods of time. Container 210 may
be back-filled with an inert gas such as Argon. Optionally,
container 210 may be back-filled with forming gas comprising 95%
nitrogen and 5% hydrogen. Note that the forming gas alone is not
sufficient to reduce a lithium tantalate material so that its bulk
conductivity is increased. However, in the present example, forming
gas helps in trapping oxygen that may have remained in container
210 after step 404. Back-filling container 210 with forming gas may
not be needed in applications where container 210 has been
completely purged of contaminants. Container 210 may be back-filled
by welding plug 215 to tube section 214, breaking the cap off tube
section 213, and then flowing back-fill gas through tube section
213.
[0041] In step 408, container 210 is sealed. At this point,
container 210 may be sealed by removing the source of the back-fill
gas and capping tube section 213. (Note that end-cap 212 has
already been welded onto body 211 and tube section 214 has already
been capped in previous steps.)
[0042] In step 410, container 210 is inserted in housing 220.
[0043] In step 412, housing 220 is heated in process tube 310 at a
temperature below the Curie temperature of wafers 201. Heating
housing 200 at a temperature below the Curie temperature of wafers
201 melts metal 202 without substantially degrading the
ferroelectric properties of wafers 201. Melting metal 202 results
in metal vapor surrounding wafers 201. In this example, the metal
vapor comprises zinc vapor and wafers 201 are of lithium tantalate.
The interaction between zinc vapor and lithium tantalate that the
inventors believe causes the bulk conductivity of wafers 201 to
increase has been previously described above.
[0044] In one embodiment, housing 220 is heated in the middle of a
process tube 310 that is 72 inches long. Also, as shown in FIG. 3,
housing 220 may be placed in process tube 310 such that open-end
221 is facing door 301. Container 210 is preferably placed inside
housing 220 such that end-cap 212 is towards open-end 221 (see FIG.
2).
[0045] In one embodiment, housing 220 is heated in process tube 310
at a ramp up rate of about 150.degree. C./hour to a maximum
temperature of about 595.degree. C., for about 240 hours.
Preferably, housing 220 is heated to a maximum temperature just a
few degrees below the Curie temperature of wafers 201. Because the
Curie temperature of wafers may vary depending on their
manufacturer, the maximum heating temperature may have to be
adjusted for specific wafers. The heating time of housing 220 in
process tube 310 may also be adjusted to ensure adequate
indiffusion of the metal vapor. Note that because method 400 is
performed on bare wafers 201 (i.e., before devices are fabricated
on wafers 201) the total process time of method 400 does not
appreciably add to the amount of time needed to fabricate a
device.
[0046] Continuing in step 414, the temperature inside process tube
310 is ramped down to prevent the just processed wafers 201 from
being degraded by thermal shock. In one embodiment, the temperature
inside process tube 310 is ramped down by setting its temperature
set point to 400.degree. C. Thereafter, cantilever 302 (see FIG. 3)
may be programmed to move housing 220 towards door 301 at a rate of
about 2 cm/minute for 3 minutes, with a 1.5 (one and a half) minute
pause time between movements. That is, housing 220 may move at a
rate of 3 cm/minute for 3 minutes, then pause for 1.5 minutes, then
move at a rate of 3cm/minute for 3 minutes, then pause for 1.5
minutes, and so on for a total of 40 minutes until housing 220
reaches door 301.
[0047] As housing 220 is moved towards door 301, open-end 221 of
housing 220 becomes cooler than closed-end 224. This results in a
thermal gradient inside container 210, with end-cap 212 (which
along with open-end 221 is facing door 301) becoming colder than
the rest of container 210. The creation of a thermal gradient in
container 210 may also be facilitated by adjusting the heaters of
process tube 310 such that the temperature is lower towards door
301. The thermal gradient inside container 210 results in end-cap
212 becoming a cold spot that attracts precipitating metal vapor
away from wafers 201.
[0048] In step 416, housing 220 is removed from process tube 310.
Container 210 is then removed from housing 220.
[0049] In step 418, wafers 201 are removed from container 210. Step
418 may be performed by first cracking open tube section 213 (see
FIG. 1) to slowly expose container 210 to atmosphere. Container 210
may also be back-filled with an inert gas. Thereafter, end-cap 212
may be cut away from body 211 using a diamond-blade saw, for
example.
[0050] In step 420, wafers 201 are polished to remove precipitates
from their surface and to expose their bulk. In one embodiment,
both sides of a wafer 201 are polished by chemical-mechanical
polishing to remove about 50 microns from each side.
[0051] In an experiment, five 42 degree rotated-Y lithium tantalate
wafers that are 100mm in diameter, hereinafter referred to as
"experimental wafers", were processed in accordance with the just
described method 400. The experimental wafers were placed in a
container 210 along with 8 grams of zinc, and then heated in a
process tube 310 to 595.degree. C. for 240 hours. Thereafter, the
temperature of the process tube 310 was ramped down and the
experimental wafers were removed from the container 210. The
experimental wafers were then polished on both sides and visually
inspected. The experimental wafers looked homogenous and grayish in
color. The bulk conductivity of the experimental wafers was then
tested by placing them one at a time on a hot plate, raising the
temperature of the hot plate from 80.degree. C. to 120.degree. C.
at a rate of 3.degree. C./min, and measuring the resulting electric
field near the surface of the wafers. The electric field was
measured using an electrometer from Keithley Instruments of
Cleveland, Ohio under the model name Model 617. The experimental
wafers did not produce any measurable electric field near their
surface, indicating that their bulk conductivity has increased.
[0052] For comparison purposes, an unprocessed 42 degree rotated-Y
lithium tantalate wafer that is 100 mm in diameter, referred to
herein as a "control wafer", was placed on a hot plate. The
temperature of the hot plate was then increased from 80.degree. C.
to 120.degree. C. at a rate of 3.degree. C./min. Measuring the
electric field near the surface of the control wafer indicated a
400V increase for every 20.degree. C. change in temperature. This
indicates that the bulk conductivity of the control wafer is
relatively low.
[0053] FIG. 5 shows a schematic diagram of a wafer cage 203A in
accordance with an embodiment of the present invention. Wafer cage
203A is a specific implementation of wafer cage 203 shown in FIGS.
1 and 2. Wafer cage 203A may be employed in the process of method
400 or method 1000, which is later discussed in connection with
FIG. 10. It should be understood, however, that wafer cage 203A is
not so limited and may also be employed in other wafer processing
applications. Furthermore, method 400 and method 1000 are not
limited to the use of wafer cage 203, wafer cage 203A or the other
apparatus disclosed herein. Methods 400 and 1000 may be performed
using different wafer processing apparatus without detracting from
the merits of the present invention.
[0054] Wafer cage 203A comprises a process boat 510 and a shell
comprising a top portion 521 and a bottom portion 522. Boat 510
comprises U-pieces 511 (i.e., 511-1, 511-2), bar pieces 512 (i.e.,
512-1, 512-2), and rods 513 (i.e., 513-1, 513-2, 513-4). Rods 513
and U-pieces 511 form a structure for holding one or more wafers in
boat 510. Rods 513 may have one or more notches (see FIG. 6), with
each notch having a width that is wide enough to receive a single
wafer. Wafer cage 203A may be made of quartz, for example. In that
case, a laser may be employed to machine the notches on rods
513.
[0055] Bottom portion 522 of the shell includes clearances 526
(i.e., 526-1, 526-2, 526-3, 526-4). Each of clearances 526 forms a
hole with a corresponding clearance 527 (i.e., 527-1, 527-2, 527-3,
527-4) of top portion 521. That is, when top portion 521 is placed
over bottom portion 522, clearances 526-1 and 527-1 form a hole,
clearances 526-2 and 527-2 form another hole, and so on. Clearances
527-3 and 527-4 of top portion 521 are not visible in FIG. 5.
[0056] Boat 510 may be placed and secured in bottom portion 522 by
having bars 512 rest on clearances 526. For example, boat 510 may
be placed in bottom portion 522 such that the ends of bar 512-1
settle on clearances 526-2 and 526-3, and the ends of bar 512-2
settle on clearances 526-1 and 526-4. Bars 512 may stick out of
clearances 526 to allow an operator to readily pick-up boat 510 by
the ends of bars 512. Top portion 521 goes over bottom portion 522
to enclose boat 510. Top portion 521 includes prongs 524 (one of
which is not shown) that go into sockets 525 (one of which is not
shown) of bottom portion 522 when the two portions are joined
together to enclose boat 510.
[0057] When employed in a process where wafers are to be exposed to
metal vapor (e.g., methods 400 and 1000), the shell advantageously
helps contain metal vapor in the vicinity of the wafers during the
main step of the process. During a temperature ramp down at the end
of the process, however, metal vapor may turn into precipitates
that may form on the surface of the wafers. The shell includes
slots 523 to advantageously minimize the formation of precipitates
on the wafers. During a temperature ramp down, the shell cools
faster than the wafers enclosed therein, thereby attracting metal
vapor to escape out of the shell and away from the wafers through
slots 523. Slots 523 also prevent excessive pressure build-up
within the shell.
[0058] FIG. 6 shows a manufacturing specification for a process
boat in accordance with an embodiment of the present invention.
FIG. 6 is for a specific implementation of boat 510. In the example
of FIG. 6, the rods have 25 notches to accommodate 25 wafers. The
boat of FIG. 6 may accommodate additional wafers by decreasing the
pitch between notches. For example the pitch may be decreased to
accommodate 50 wafers. The length of the rods may also be
lengthened to accommodate more wafers. The dimensions in the
example of FIG. 6 are in inches unless otherwise indicated.
[0059] FIG. 7 shows a manufacturing specification for a shell in
accordance with an embodiment of the present invention. FIG. 7 is
for a specific implementation of the shell comprising top portion
521 and bottom portion 522 shown in FIG. 5. In the example of FIG.
7, the dimensions are in inches unless otherwise indicated.
[0060] FIG. 8 shows a schematic diagram of a container 210A in
accordance with an embodiment of the present invention. Container
210A is a specific implementation of container 210 shown in FIG. 1.
Container 210A is the same as container 210 except for the addition
of a nipple 801 in end-cap 212A. Reference labels common between
FIGS. 1 and 8 indicate the same or similar components. Container
210A may be made of quartz, for example. Note that container 210A
and other apparatus disclosed herein may be made of a material
other than that disclosed without detracting from the merits of the
present invention. Those of ordinary skill in the art will be able
to select materials for the disclosed apparatus to meet the needs
of specific applications.
[0061] As shown in FIG. 8, cage 203A may be used within container
210A.
[0062] FIG. 9 shows a system 900 for increasing the bulk
conductivity of a ferroelectric material in accordance with an
embodiment of the present invention. System 900 is the same as
system 300 shown in FIG. 3 except for the use of container 210A
instead of container 210. Reference labels common between FIGS. 3
and 9 indicate the same or similar components. In one embodiment,
system 900 does not include a housing enclosing container 210A.
Wafers to be processed and a metal source (e.g., zinc pellets) may
be placed in cage 203A, which in turn may be placed in container
210A.
[0063] FIG. 10 shows a flow diagram of a method 1000 for processing
a ferroelectric material in accordance with an embodiment of the
present invention. Method 1000 will be described using system 900
as an example, not a limitation.
[0064] In step 1002, a metal source and one or more wafers are
placed in container 210A. Step 1002 may be performed by placing the
wafers in boat 510, placing boat 510 and the metal source in bottom
portion 522, covering bottom portion 522 with top portion 521, and
then placing the resulting assembly (i.e., cage 203A) in body 211
of container 210A.
[0065] In step 1004, end-cap 212A of container 210A is welded onto
body 211 to enclose cage 203A. Step 1004 may be performed by
capping tube section 213 (see FIG. 8), leaving nipple 801 open, and
flowing nitrogen gas into tube section 214 and out through nipple
801 during the welding process. The nitrogen gas serves as a drying
agent that purges water vapor generated by the welding process out
of container 210A.
[0066] In step 1006, container 210A is pumped down. Step 1006 may
be performed by capping nipple 801, keeping tube section 213
capped, and coupling a pump to tube section 214. Container 210A
does not have to be heated during step 1006. Pumping down container
210A helps remove oxygen sources, water, and other contaminants out
of container 210A. Container 210A may be pumped down until the
pressure within it has stabilized. In one embodiment, container
210A is pumped down for about 5 minutes.
[0067] In step 1008, container 210A is back-filled so that the
pressure in container 210A at slightly below Curie temperature is
approximately 760 Torr. Container 210A may be back-filled with an
inert gas such as argon. Optionally, container 210A may also be
back-filled with forming gas to trap oxygen that may have remained
in container 210A after step 1006. Container 210A may be
back-filled by welding plug 215 to tube section 214, breaking the
cap off tube section 213, keeping nipple 801 capped, and then
flowing back-fill gas through tube section 213.
[0068] In step 1010, container 210A is sealed. Container 210A may
be sealed by removing the source of the back-fill gas, capping tube
section 213, keeping tube section 214 capped, and keeping nipple
801 capped.
[0069] In step 1012, container 210A is placed in process tube 310
of system 900 (see FIG. 9). Container 210A may be placed in the
middle of process tube 310, which in the example of FIG. 9 is the
heating zone heated by heater 303B. Container 210A may be placed in
process tube 310 at room temperature. Note that container 210A may
be placed inside process tube 310 without a housing.
[0070] In step 1014, process tube 310 is prepared to run the
process. Step 1014 may be performed by starting the flow of a
nitrogen gas in the furnace. The nitrogen gas may be flowed
continuously during the process run. at a flow rate of about 5
liters/min. The nitrogen gas helps preserve the integrity of
components made of quartz, such as container 210A in this
example.
[0071] In step 1016, the temperature inside process tube 310 is
ramped up. In one embodiment, the temperature inside process tube
310 is ramped up at a rate of about 2.5.degree. C./min to about
595.degree. C. Depending on the particulars of the process tube
employed, heaters 303A, 303B, and 303C may be configured such that
the temperature in the middle section of the process tube where
container 210A is placed is maintained at a target temperature
(about 595.degree. C. in this example) that is below a Curie
temperature.
[0072] In step 1018, the temperature inside process tube 310 is
allowed to stabilize. Step 1018 may be performed by waiting for
about 25 minutes before proceeding to step 1020.
[0073] In step 1020, container 210A is heated for a target amount
of time at a target temperature. The target temperature is
preferably slightly below the Curie temperature of the wafers being
processed, while the target amount of time may be varied to achieve
a target wafer conductivity. For example, container 210A may be
heated at a temperature of about 595.degree. C. for about 25 hours
or less. The inventors believe that heating time is proportionally
related to bulk conductivity. That is, the longer the heating time,
the higher the resulting bulk conductivity of the wafers. For
example, a heating time of about 200 hours may result in the wafers
having a bulk conductivity of about 10.sup.-10(.OMEGA.cm).sup.-1,
while a heating time of about 25 hours may result in the wafers
having a bulk conductivity of about 10.sup.-12(.OMEGA.cm).sup.-1.
For comparison purposes, an unprocessed wafer may have a bulk
conductivity of about 10.sup.-16(.OMEGA.cm).sup.-1. The heating
time may thus be varied to meet the conductivity requirement of
specific applications.
[0074] In step 1022, the temperature inside process tube 310 is
ramped down to prevent the wafers from being degraded by thermal
shock. In one embodiment, step 1022 is performed by ramping down
the temperature in all heating zones of process tube 310 to about
530.degree. C. at a rate of about 1.5.degree. C./min.
[0075] In step 1024, container 210A is pulled out of process tube
310. In one embodiment, container 210A is pulled out of process
tube 310 at a rate of about 3 cm/min using the following
sequence:
[0076] a) pull container 210A out 15 cm, wait 1 minute;
[0077] b) pull container 210A out 15 cm, wait 1 minute;
[0078] c) pull container 210A out 10 cm, wait 1 minute and 10
seconds;
[0079] d) pull container 210A out 10 cm, wait 1 minute and 10
seconds;
[0080] e) pull container 210A out 10 cm, wait 1 minute and 10
seconds;
[0081] f) continue pulling out container 210A at 10 cm increments
until 90 cm has been covered;
[0082] g) pull container 210A out the remaining distance to the
opening of process tube 310 at a rate of 3 cm/min.
[0083] In step 1026, the wafers are removed from container 210A
after container 210A has cooled down. The wafers may be wet etched
or polished to remove precipitates that may have formed on their
surface and to expose their bulk.
[0084] While specific embodiments of the present invention have
been provided, it is to be understood that these embodiments are
for illustration purposes and not limiting. Many additional
embodiments will be apparent to persons of ordinary skill in the
art reading this disclosure. Thus, the present invention is limited
only by the following claims.
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